Transmitter-specific subsets of sensory elements in the prosobranch osphradium.

The osphradium is a putative chemosensory organ of aquatic molluscs. Previously, we identified two distinct types of primary sensory neurons in the osphradial ganglion of freshwater pulmonates, one immunoreactive to leucine-enkephalin (LEnk-ir) and another to FMRFamide (FMRFa-ir). In addition, NADPH diaphorase (NADPHd)-positive elements apparently producing nitric oxide (NO) were demonstrated in the organ. In the present study, prosobranch molluscs, which have retained the osphradial sensory neurons within the epithelium, were studied. Both types of peptidergic neurons, as well as NADPHd-positive cells, were found within the epithelium or in a basiepithelial position in the relatively simple osphradium of the mesogastropod Littorina littorea and in the complex, bipectinate osphradium of the neogastropod Buccinum undatum. Similar evidence was also obtained for another mesogastropod, Ampullarius sp. Transmitter-specific sensory cell types like those discovered in the osphradium are also present as single neuroepithelial cells in other organs of the mantle complex in prosobranchs and in the pelecypod Anodonta cygnea. We suggest that evolutionarily conservative, transmitter-specific types of epithelial and neuroepithelial sensory cells predated the osphradium, which developed as the site of their concentration, while retaining characteristic subsets of sensory neurons.